I suspect the most dangerous habit of thought taught in schools is that even if you don't really understand something, you should parrot it back anyway. One of the most fundamental life skills is realizing when you are confused, and school actively destroys this ability — teaches students that they "understand" when they can successfully answer questions on an exam, which is very very very far from absorbing the knowledge and making it a part of you. Students learn the habit that eating consists of putting food into mouth; the exams can't test for chewing or swallowing, and so they starve.